Sexual and parasexual genetic analyses have permitted the detection, characterization and genetic localization of a number of mammalian cellular genes which participate in neoplastic transformation and retroviral expression. Seven classes of genes are under study: (1) endogenous cellular DNA sequences homologous to retroviral RNA; (2) integration sites of retroviruses in mammalian chromosomes; (3) growth factor receptors; (4) restriction genes which delimit viral replication; (5) enzyme structural genes; (6) cell surface antigens including major histocompatibility (MHC); and (7) cellular transforming genes (onc) rescued by defective retroviruses. Examples of each of these gene classes have been detected and mapped in human and feline model systems. Highlights include the genetic analysis of the ras gene family in man and cat; the genetic analysis of two families of endogenous retroviral loci in man (ERV1+), genetic mapping of the human homologous loci of the raf-1 and raf-2 murine oncogenes; genetic analysis of the RD-114 family in the cat; and genetic localization of 10 independent integration sites of HTLV in human T cell leukemias.